PARIS—Serena Williams, the winner of 23 Grand Slam titles, often dominates tennis’ most coveted events, leaving her young challengers with scraps. Now that she’s pregnant and skipping the French Open, Williams has created a different problem: No player has any idea what to do in a wide open field.

“I’ve never seen a time like this,” said Pam Shriver, the former pro and ESPN commentator. “I thought I’d seen some draws in the last 10 years that were wide open on the women’s side, but I’ve never seen a situation like this.”